1. Field of the Invention
This invention relates in general to the art of coating and in particular to the application of liquid coating compositions to a spliced web. More specifically, this invention relates to an improved spliced web and to the utilization of such web in a coating process to achieve a reduction in coating disturbances caused by the splice during coating of the spliced web with a liquid composition.
2. Description of the Prior Art
A longstanding problem in the art of coating web materials with liquid coating compositions is disturbance of the coating operation that is caused by splices in the web. The problem is particularly acute in the photographic art in which it is very common to make use of the bead coating technique to coat a web of photographic support material with liquid photographic coating compositions, such as radiation-sensitive gelatin silver halide emulsions utilized to form imaging layers or other hydrophilic colloid compositions employed in subbing layers, inter-layers, antistatic layers, protective overcoats layers, and so forth. In the bead coating process, one or more liquid layers are fed into a coating bead which is maintained in bridging relationship between the surface of a moving web and a coating device which is closely spaced therefrom and the surface of the web is continuously moved across and in contact with the coating bead to pick up the one or more liquid layers. The coating bead utilized in this method of coating is very susceptible to disturbance by the splice, which is typically a butt-splice in which web portions in contiguous end-to-end relationship are joined together by a splicing tape. A face-side splice is generally utilized, that is, the splicing tape is on the surface of the web to which the coating composition is applied, since this generally results in less disturbance than a back-side splice.
Disturbance of the coating bead by splices causes serious defects in the coating. The problem occurs even at low or moderate coating speeds but becomes especially severe at high coating speeds. Among the difficulties commonly encountered are the tendency of the coating bead to break into distinct areas, causing regions of excessively heavy coating and regions of no coating on the support. A further serious problem is the trapping of air bubbles at the trailing edge of the splicing tape. (The terms "leading edge" and "trailing edge" of the splicing tape are used herein to distinguish the two edges in relation to the direction of travel of the web, with the "trailing edge" being the edge on the downstream side and the "leading edge" the edge on the upstream side). Trapped air bubbles cause problems in a number of ways. For example, they may become lodged in the coating bead or on the lip of the coating hopper and cause a longitudinal streak in the coating which can render worthless substantial quantities of the coated material. Where the coating operation involves coating at two or more stations in succession, bubbles introduced into the coating at one station can be sheared off at a subsequent station and thereby cause the formation of streaks. Moreover, bubbles in the coating can result in incomplete drying so that at the first instance where the coated layer comes into contact with a roller there is a tendency for coating composition to adhere to the roller, resulting in a coating defect referred to as "track-off."
The leading edge of the splicing tape is not ordinarily an important factor in causing coating disturbances. Such disturbances are primarily associated with the trailing edge. In particular, the trapping of air bubbles occurs at the trailing edge as the coating bead passes over the splicing tape and then returns to engagement with the web surface. A major consideration in regard to the extent to which bubbles will be trapped and the coating operation will be disturbed is the magnitude of the vertical drop at the trailing edge of the splicing tape, that is, the distance from the surface of the splicing tape to the adjacent web surface. In general, the greater the extent of this vertical drop the greater the degree of disturbance of the coating process.
There have been many efforts made in the past to alleviate the problems associated with coating over splices and to reduce the extent to which disturbance of the coating operation takes place. For example, a simple method for reducing coating disturbances caused by splices is described in U.S. Pat. No. 3,518,141 and British Pat. No. 1,243,663. In this method, the trailing edge of the splicing tape and the adjacent web surface is covered with a film of water prior to the coating operation and the coating composition is applied before the water dries. Certain other techniques for reducing coating disturbances at splices are described in U.S. Pat. No. 3,531,362. The techniques described involve coating the trailing edge of the splicing tape and the adjacent web surface with a hydrophobic material, feathering or beveling the trailing edge of the splicing tape, and filling in the transition from the tape to the web surface with a suitable filler material such as rubber cement. A reduction in coating disturbances caused by splices can also be provided, as described in U.S. Pat. No. 3,916,043, by control of the differential pressure which is maintained to stabilize a coating bead. Thus, the disturbances caused by splices are greatly decreased if the differential pressure is increased to an elevated level just before a splice enters the coating station, is maintained at this level during the time the splice is passing the coating station and for a short time thereafter, and is then reduced to the normal level. Yet another procedure for reducing coating disturbances caused by splices is that described in U.S. Pat. No. 3,972,762 in which a portion of the web is "preworked" so that the web surface immediately downstream of the trailing edge of the splicing tape is at least coplanar with the surface of the splicing tape.